1. Field of the Invention
The present invention relates to a terminal supporting multiple modes, and more particularly to a method for efficiently acquiring a handover network and a multi-mode terminal therefor, which can minimize a period of time required for acquiring the handover network between communication networks that provide communication services using different mobile communication technologies.
2. Description of the Related Art
Currently, mobile communication technology is evolving from the Code Division Multiple Access (CDMA) and European GSM (Global System for Mobile communication) systems, which are 2nd generation network systems, into a 3rd generation network system, such as a Wideband CDMA (WCDMA) system. Since there exists a vast and well-equipped service area using the 2nd generation communication scheme, the service area using the 3rd generation communication scheme is being extended even broader while the base facility for the 2nd generation communication scheme is being utilized, during this time of transition. Therefore, a terminal designed to receive a service specified in the 3rd generation network during transition is produced as a multi-mode terminal, which can support the existing 2nd generation communication scheme in addition to the 3rd generation communication scheme.
Such multi-mode terminals include an idle handover function for performing a mode transition between a WCDMA mode and a CDMA mode in an idle state, and a traffic handover function for performing a mode transition to a CDMA mode without discontinuance of communication when the terminal enters a shadow area from a WCDMA network during communication. These functions are economical since they utilize the existing CDMA network in the 2nd generation system, but have a disadvantage of needing complicated hardware and software in the terminal, due to the need for quick handover between different modems.
The operation of a terminal, which performs a handover between the two modes during communication, will now be described with reference to FIG. 1. In order to perform a handover to a CDMA mode during communication in a WCDMA mode, both WCDMA modem and CDMA modem included in the terminal are powered on so as to transmit a control signal required for the handover. FIG. 1 shows the construction of a dual-mode terminal, which has a single antenna structure and supports the CDMA and WCDMA modes. In order to support the WCDMA and CDMA modes, the terminal uses modem chips which support different mobile communication services depending on the modes. When the terminal having such a construction is located in a WCDMA network, a CDMA RF unit 220 and a CDMA modem 240 are temporarily powered off, and the single antenna, a duplexer 210, a WCDMA RF unit 230 and a WCDMA modem 250 are powered on, so that the terminal operates in the WCDMA mode. Conversely, when the terminal having such a construction is located in a CDMA network, the WCDMA RF unit 230 and the WCDMA modem 250 are temporarily powered off, and the single antenna, the duplexer 210, the CDMA RF unit 220, and the CDMA modem 240 are powered on, so that the terminal operates in the CDMA mode.
To this end, the modems 240 and 250 are individually connected to a controller 200 through a switch 260, and one modem is powered off while the other modem is operating so as to prevent waste of power. The modems 240 and 250 are connected to each other through an interface 270 for mutual communication.
Meanwhile, when a handover of such a terminal occurs, the terminal must perform a procedure in order to acquire a network, to which the terminal is to be handed over. The procedure will now be described with reference to FIG. 2
Referring to FIG. 2, when the user attempts to make communication in a WCDMA mode, a controller 200 establishes a voice communication path through a WCDMA modem 250 in step 200 so that voice communication can be performed. A related communication connection procedure is well-known in the art, so a detailed description thereof will be omitted. When communication begins, the controller 200 switches on a CDMA modem 240 to enable the CDMA modem 240 to enter a low power mode (step 205).
Thereafter, a procedure for determining whether the current radio environment corresponds to a handover condition is performed between the WCDMA modem 250 and a WCDMA base station 160 in steps 210 and 215. When a handover occurs, the WCDMA modem 250 receives a Handover from Umts Terrestrial Radio Access Network (UTRAN) Command, which instructs a handover into the CDMA mode, in step 220. Then, the WCDMA modem 250 extracts a Universal Handover Direction Message (UHDM) by analyzing the received command, and transmits the UHDM to the CDMA modem 240.
Meanwhile, separate from the operation of the WCDMA modem 250, when the CDMA modem 240 receives the UHDM while in the low power mode, the CDMA modem 240 regards the UHDM as a command to acquire a CDMA network. However, since the CDMA modem 240 has not yet acquired a CDMA network, the CDMA modem 240 cannot perform a handover based on the UHDM. Therefore, when the UHDM has been received, the CDMA modem 240 regards the UHDM as a command to perform an operation for acquiring a system, to which the terminal is handed over, without instantly processing the UHDM. Then, the CDMA modem 240 enters a service mode to analyze a Preferred Roaming List (PRL) in step 230, and performs an operation for acquiring a CDMA network based on a result of the analysis in step 235. When CDMA network acquisition based on the analysis result is successful, the CDMA modem 240 parses the received UHDM to extract channel information for a handover. Accordingly, the CDMA modem 240 can complete the CDMA network acquisition of step 235, by again setting a handover channel based on the extracted channel information for a handover. That is, regardless of reception of the UHDM, the CDMA modem 240 must acquire a network using a PRL before setting a handover channel using the UHDM, so that a network acquisition procedure is performed twice.
The operation performed in steps 230 and 235 will now be described in more detail. The CDMA modem 240 uses a PRL in order to acquire a CDMA network. Herein, the PRL stores CDMA frequencies and bands, which can be acquired by the terminal, and is constructed with all channels capable of being provided by each network provider.
Therefore, the terminal must acquire a CDMA network based on the channel sequence of the PRL. In this case, if a channel of an actual radio environment corresponds to a channel located in the upper portion of the PRL, it is possible to rapidly acquire a corresponding channel. However, in contrast, if a channel of an actual radio environment corresponds to a channel located in the lower portion of the PRL, it takes a long period of time to acquire a CDMA network because the terminal searches channels stored in the PRL one by one according to the sequence thereof.
As described above, according to the conventional traffic handover from a WCDMA network to a CDMA network, although having received the UHDM from a WCDMA modem, the CDMA modem only recognizes the received message as a command to start CDMA network acquisition. This is because a handover procedure of the CDMA modem based on the UHDM is performed after a CDMA network has been acquired. While the CDMA modem performs CDMA network acquisition according to the sequence of channels in the PRL, if a CDMA channel of an actual radio environment corresponds to a channel located in the rear portion of the PRL, it takes a long period of time to acquire a CDMA network, so that much more time is necessary to process a handover. As described above, after having acquired a CDMA network, the CDMA modem must parse the received UHDM, and must again establish a handover channel based on information about the handover channel extracted through the parsing operation, so as to complete the CDMA network acquisition. That is, a channel establishment procedure for a handover must be performed twice.